Aluminum carbon composite seal material

ABSTRACT

A process for impregnating a mass of carbon particles with metal, preferably an aluminum alloy, to form a solid billet suitable for machining therefrom articles having advantageous durability and lubricity characteristics, particularly as apex seals for Wankel engines. A mass of carbon particles is heated to a temperature at or above the melting point of the metal in a closed crucible of permeable refractory material such as porous carbon, and the molten metal is poured over the porous crucible into a second crucible wherein the porous crucible becomes enveloped by the molten metal but the molten metal does not penetrate the porous crucible. The atmosphere surrounding the molten metal and porous crucible is then pressurized to force the molten metal through the porous crucible and thus cause controlled impregnation of the mass of particles.

United States Patent 1 Demendi 1 ALUMINUM CARBON COMPOSITE SEAL MATERIAL[75] Inventor: Joseph F. Demendi, St. Marys, Pa.

[73] Assignee: Pure Carbon Company, Inc., St.

Marys, Pa.

[22] Filed: Oct. 16, 1974 [21] Appl. No.: 515,327

Related U.S. Application Data [62] Division of Ser. No. 299,048, Oct.19, 1972, Pat. No.

[52] U.S. C1. 29/l9l.2; 75/142; 75/143; 75/148; 164/97 [51] Int. Cl.B23P 3/00 [58] Field of Search 29/191.2, 180 R; 75/138 US, 148 US, 142,143; 164/97 [56] References Cited UNITED STATES PATENTS 2,793,949 5/1957Imich 75/138 3,147,087 9/1964 Eisenlohr. 29/1912 3,239,319 3/1966Pollard 75/148 X 3,262,762 7/1966 Bechtold 75/148 X 3,306,738 2/1967Young et a1. 75/148 X 3,333,579 8/1967 Shockley et a1. 75/148 X3,600,163 8/1971 Badia 75/138 51 Sept. 23, 1975 Primary ExaminerFrancisS. Husar Assistant Examiner-V. K. Rising Attorney, Agent, orFirm-Bosworth, Sessions & McCoy [57] ABSTRACT A process for impregnatinga mass of carbon particles with metal, preferably an aluminum alloy, toform a solid billet suitable for machining therefrom articles havingadvantageous durability and lubricity characteristics, particularly asapex seals for Wankel engines. A mass of carbon particles is heated to atemperature at or above the melting point of the metal in a closedcrucible of permeable refractory material such as porous carbon, and themolten metal is poured over the porous crucible into a second cruciblewherein the porous crucible becomes enveloped by the molten metal butthe molten metal does not penetrate the porous crucible. The atmospheresurrounding the molten metal and porous crucible is then pressurized toforce the molten metal through the porous crucible and thus causecontrolled impregnation of the mass of particles.

1 Claim, 4 Drawing Figures MOLTEN ALLOY Sept. 23,1975

POROUS CARBON CRUCIBLE US Patent OOOQOB 1 m mm O 2 V 5 E 1/ w U 6 R B Cn \A AMH H H A l C 1 Y6 N ma NA um U WF BILLET FIG. 4

ALUMINUM CARBON COMPOSITE SEAL MATERIAL This is a division ofapplication Ser. No. 299,048, filed Oct 19, 1972, now US. Pat. No.3,853,635.

BACKGROUND OF THE INVENTION This invention relates to the manufacture ofa seal material for use in sealing moving parts of expansion chamberssuch as the combustion chambers of rotary piston internal combustionengines. More particularly, the invention relates to a method for makinga billet or blank of a composite carbonaluminum alloy material havingadvantageous properties, from which seals such as apex seals may bemachined.

The apex seals for rotary piston engines (e.g., Wankel engines) arefitted at each apex of the rotary piston or trochoid and are adapted tomake a planetary motion with the piston while being urged against theinner surface of the stator by the combined action of the elastic forceof springs disposed behind the apex seal, gas pressure in the operatingchamber and centrifugal force produced by the rotation and orbiting ofthe piston. The seal moves in sliding contact with the inner surface ofthe stator and maintains an airtight seal between adjacent expansionchambers on opposite sides of the apex. Therefore, the apex seal usedfor this pur pose must have excellent mechanical strength at alltemperatures encountered and sufficient lubricity that it does notproduce an excessive wear on the inner surface of the stator. In view ofthese considerations and others, the quality of the apex seal has asignificant bearing on the performance and durability of the enginc.

Many and various materials have been proposed for the apex seal such asthose shown in US. Pat. No. 3,619,430 and in British Pat. No. 1,234,634.The most successful material presently available comprises carbonparticles impregnated with an aluminum silicon alloy.

Carbon is an excellent bearing material due to its heat resistance, wearresistance and corrosion resistance as well as other advantages such ashigh thermal conductivity and low thermal expansion. Accordingly, carbonin various forms and combinations is used for many types of mechanicalelements, particularly sliding members in view of its wear resistanceand lubricity. The use of carbon products as sliding members and thelike, however, is limited because of their inherently low durability,particularly where they are subjected to considerable vibration andimpact.

Methods have been devised for manufacturing apex seals of a mass ofcarbon particles impregnated with a continuous phase of aluminum or analuminum alloy and while aluminum is somewhat inferior in wearresistance to many other metals and has a relatively low melting point,the resulting composite material has increased strength and betterresistance against vibration and impact. One reason that thealuminumimpregnated carbon type apex seal has superior characteristicsis that a high ratio of impregnation can be achieved with aluminum andthe aluminum in the resulting product is intimately and tightly bondedto the carbon due to the formation of aluminum carbide at thecarbonaluminum interface.

One impregnating process is accomplished in an autoclave by immersing aporous press-molded blank made from carbon particles and a binder in amolten aluminum or aluminum alloy bath. The autoclave is evacuated andthe pre-pressed blank is impregnated with the molten metal in a nitrogenor argon atmosphere in order to prevent the carbon from oxidizing.Following immersion of the carbon blank a high pressure is applied toforce the molten metal into the voids in the carbon blank. For suitableproperties, it is necessary that the impregnation of the carbon blankwith the alloy be as complete as possible. It has been found that asufficient impregnation of metal is difficult to achieve without usinghigh temperatures and pressures over relatively long times.

An autoclave and related equipment for achieving the impregnation isdisclosed in US. Pat. No. 3,599,601. Using the apparatus shown in thatpatent, a sintered or otherwise preformed molded part is loaded in a dipcage formed of refractory material having a plurality of slits formedtherein to permit entry of mol ten metal. The product is heated in thedip cage to the desired temperature and then with the autoclavepartially evacuated, lowered into a bath of molten metal. A pressure isapplied and the molten metal is thus forced through the porous articleto be impregnated.

The method of the present invention affords an im proved process whichcan be accomplished more quickly, more efficiently, is accuratelycontrolled, eliminates the need for preforming the part or parts to beimpregnated, permits a higheralloy to carbon ratio, and affords otherfeatures and advantages heretofore not obtainable.

SUMMARY OF THE INVENTION It is among the objects of the invention toprovide an improved method for impregnating a carbonaceous product witha molten metal.

Another object is to provide an improved method for making combustionchamber seals for rotary pistontype internal combustion engines.

These and other objects are achieved through a method for impregnatingcarbon particles with a mo]- ten metal, preferably an aluminum oraluminum alloy, which includes the following steps:

l placing a quantity of carbon particles in a porous crucible formed ofrefractory material such as porous carbon and densifying the particlesby vibration or jarring,

(2). heating the carbon powder in the porous crucible to a temperaturewithin or above the melting point range of the metal in an inertatmosphere,

(3). placing the heated porous crucible and carbon particles in animpermeable crucible formed, for example, of ceramic material,

(4). heating the metal within or above its melting point range,

(5). pouring the metal in molten form over the porous crucible and intothe impermeable crucible,

(6). holding the porous crucible in the molten metal for at least halfan hour in order to establish thermal equilibrium at a temperaturewithin the melting point range of the metal.

(7). placing the two crucibles in a pressure chamber such as anautoclave, and pressurizing the atmosphere to a pressure between 2000and 4500 psi for a very short time, I to 10 mintues, to force the moltenmetal to penetrate the porous carbon crucible and impregnate the mass ofcarbon particles, and

(8). removing the porous crucible, quenching and cooling to roomtemperature and then breaking'away the porous crucible to obtain abillet of impregnated material.

When the metal is an aliminum-silicon alloy, the billet is roughmachined into blank parts and further heat treated. It should be heatedto approximately 935F. for about eight hours, then heated at about 450F.for about 4 hours, and then final machined into apex seals. Thistreatment imparts diminsional stability and the desired grain structure.When other alloys and metals are used, further annealing and heattreatment should be employed as is appropriate to the particular alloyor metal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing aninitial portion of the method of the invention using apparatus that isillustrated in diagrammatic form merely for the purpose of illustratingthe method;

FIG. 2 is a sectional view showing the apparatus in a manner similar toFIG. 1 and illustrating a subsequent step in the practice of theinvention;

FIG. 3 is still another sectional view illustrating appa ratus in amanner similar to FIGS. 1 and 2 and showing a still further step in thepractice of the method of the invention; and

FIG. 4 is a sectional view in diagrammatic form illustrating thebreaking away of the material forming the impregnated porous cruciblefrom the carbonalluminum composite produce made in accordance with themethod of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As indicated above, theinvention resides in a method for making carbon-aluminum impregnatessuitable for use in fabricating seals for various types of mechanicalequipment such as seals used in the trochoid of rotary piston-typeinternal combustion engines. The method first requires the selection ofappropriate materials. It has been found that a particularlyadvantageous form of particulate carbon material is calcined coconutchar which is a particularly hard and abrasion-resistant type of char.This material preferably has a particle size of nomo're than 100 micronsand preferably no more than about 40 microns. Other hard,abrasion-resistant chars such as those made from walnut shells or othernut shells may also be used.

The molten aluminum alloy used as the impregnate is, in accordance withthe preferred aspect of this invention, an aluminum-silicon alloy havinga silicon content ranging from to 35 percent by weight. The silicon inthe alloy forms small grains which give the alloy strength at hightemperatures. Preferably this is a hyper-eutectic alloy having 14 to 19percent silicon. Sili' con is soluble in aluminum to about ll percent byweight, after which the silicon crystallizes out as tiny grains orcrystals. A particularly advantageous alloy is an aluminumsiliconmaterial sold the Aluminum Association trade designation A-390. This isa hypereutectic alloy in the sense that it has more silicon than theeutectic point, in the case of A-390, about 17% silicon. Simple binaryalloys of aliminum and silicon are not believed to be desirable. Iprefer 2 to 5 percent copper or nickel and other elements found in A-39Oand similar alloys.

An aluminum alloy is preferred because it readily wets and penetratesthe carbon char particle powder. Other metals and alloys may be adaptedto this process such as copper or other metals listed in column 2 ofUS-Pat. No.- 3,599,601. Aluminum-magnesium alloys may be used. However,for the immediate purposes of this invention, I prefer a metal which isprincipally aluminum, by which I mean at least 50 percent aluminum byweight.

The porous baked carbon crucible preferably has a porosity of at leastabout percent of the total volume thereof. Crucibles of lowerporosities, as low as 5% by volume, may be employed but they are not assuitable because they require a longer impregnation time and becasuethey are more expensive to manufacture. The porous crucible ispreferably made of a non-graphitized carbon.

The porosity of the mass of carbon particles is of a higher order, as atleast 20 percent and preferably around 50 percent by volume. After beingplaced in the porous Crucible, the carbon particles are densified byvibration or jarring and the above porosities are to be taken after suchdensification. The preferred porosity range is to 60% by volume.

Referring more particularly to the drawings which show apparatus forpracticing the invention, in diagrammatic form only, a preferredembodiment of the method will be illustrated and described usingmaterials found to be particularly advantageous. Initially the mass ofcarbon particles, or more specifically, the calcined coconut char 10, isplaced in a porous ceramic crucible 11 preferably formed of fairlythin-walled baked carbon material and a lid 12 formed of the samematerial is placed thereon (FIG. 1). The powder is densified byvibration or jarring. The porous crucible 11 preferably has a porosityof about 20 percent by volume and the carbon powder preferably has aporosity of about percent by volume.

The crucible 11 is heated in an induction furnace to a temperature inexcess of the melting point of the metal or within the melting pointrange, in the case of the aluminum-silicon alloy, around llO0F. Theheating is done in a nitrogen or argon or other inert gas atmosphere toreduce or eliminate the oxygen. The crucible ll may be heated by itselfor it may be placed inside the ceramic crucible and the assembly heatedin the induction furnace in an inert atmosphere.

The metal 13 is also heated to a molten condition, again in thisinstance around ll00F. in an induction furnace. The heating mayadvantageously be accomplished in the same furnace and the temperatureshould be very carefully monitored. Preheating of the porous crucibleand carbon powder to that high a temperature, however, is onlypreferable. Preheating to lower temperatures, about 350F, may beemployed and is beneficial in that it drives out some gas and moisturefrom the carbon powder.

When the metal is heated, it may also be purified as is well known inthe art. In the case of an aluminumsilicon alloy, chlorine gas may bebubbled through the molten alloy to eliminate entrapped oxygen andhydrogen. Alternatively, degassing tablets may be added to the melt.Also, to control grain size, a nucleating agent such as aluminumphosphide may be added.

The volume of metal to be melted should be weighed and selected inaccordance with a predetermined amount of material needed depending uponthe volume of the mass of carbon particles 10. After the necessaryheating of the carbon particles and the metal 13 has been accomplished,the porous crucible 11 is carefully fitted into an impermeable ceramiccrucible preferably formed of graphite-clay material and having spacers16 located on the bottom thereof as indicated in FIG. 1. A lid 21provided with holes 22 is placed on the ceramic crucible 15 and over theporous crucible to keep it in place.

Referring next to FIG. 2, it will be seen that the molten alloy 13 ispoured from the ceramic crucible 14 over the lid 21 so that it overflowsdown and around the walls of the porous crucible 11 to fill the spaceswithin the impermeable ceramic crucible 15. The spacers l6 serve tosupport the porous crucible 11 above the floor of the impermeableceramic crucible 15 so that the molten metal essentially surrounds allsurfaces of the porous crucible 11.

The assembly of the impermeable ceramic crucible 15, porous carboncrucible 11, carbon particles 10 and molten aluminum 13 is thenmaintained at a temperature approximately above or within the meltingpoint range of the metal for a time sufficient to equalize thetemperature between the crucibles, and the molten metal. There is nopressure on the system and the molten metal does not penetrate theporous crucible 11. Finally, the assembly is placed in a quick-openingautoclave 17 (see FIG. 2) and a lid 18 is tightly attached to provide ahermetic seal.

After this thermal equilibration, the space within the autoclave 17 isthen pressurized using a pump 19 to provide a pressure between about2000 to 4500 psi for from I to 10 minutes and preferably 2 to 5 minutes.This is a pneumatic pressure, air or an inert gas such as nitrogen orargon. A pressure of 4200 psi is believed to be particularlyadvantageous. The high pressure in the autoclave atmosphere forces themolten metal 13 to penetrate the porous carbon crucible l1 and lid 12and then to impregnate the mass of carbon particles 10. Any scum thatmay form on the surface of the molten metal will be filtered out by theporous crucible 11 and thus not penetrate into the mass of carbonparticles. The extent of impregnation of the voids or spaces within themass of carbon particles 10 is essentially complete and any gasoriginally contained within the mass of particles is either dissolvedinto the molten metal or expelled through the porous crucible 11.

The application of pressure is continued for a short period of time,preferably two to five minutes, just barely sufficient to cause completeimpregnation of the mass of particles 10. As soon as the impregnation isbelieved to be complete, the pressure is reduced to nor mal, the lid 18removed and the assembly within the autoclave removed and quenched toroom temperature.

The duration of the application of pressure is minimized so as tocarefully control the reaction of aluminum and carbon which formsaluminum carbide. While the presence of aluminum carbide in the productof amounts of around 3 to 6 percent is desirable, quantities in excessof such percentages are to be avoided for the reason that excessaluminum carbide reacts with water vapor thus causing erosion of theseal material. The reaction between the molten aluminum and carbon toform an aluminum carbide interface facilitates impregnation by themolten metal.

After quenching, the billet of impregnated material is broken away fromthe porous crucible (FIG. 4). In

the case of the aluminum-silicon alloy, the billet is then cut up intoblanks and further heat treated and annealed. The billet may be annealedfor from 6 to 10 hours at 900 to 1000F. and then from three to 5 hoursat 400 to 500F., preferably at the times and temperatures previouslynoted, and then quenched in air to room temperature. After heattreatment, the blanks are finally machined into apex seals.

As should be apparent, the process of the present invention hasparticular utility in the manufacture of aluminum silicon alloyimpregnated seals. The A-390 alloy solidifies within a range of 1050 to1200F.

In dealing with such alloys, the temperature of impregnation, by which Imean the temperature at which the air pressure is introduced into theautoclave, should be within the melting point range, about I 100F. Whenthe temperature is much over 1 100F. and towards the top end of themelting point range, there is too much of a reaction between thealuminum and carbon resulting in the formation of too much aluminumcarbide and a part of poor physical properties.

While pressure in the order of 3500 to 4500 psi are preferred, I am ableto impregnate a powder with pressures down as low as 100 psi. When thereis no problem of chemical reactions between the metal and carbon powder,I believe that such lower pressures may he satisfactory for productionoperations. I prefer the higher pressures in the case ofaluminum-silicon alloys because I can obtain complete impregnation inshorter times, more surely in production.

The following example illustrates the invention:

Calcined coconut shell charcoal was placed in a porous carbon crucible.The material had the following particle size range:

671 above 32 micron 50% above 20 micron above 8 micron It was vibratedto a density of .8 grams per cubic centimeter.

Aluminum Association A-390 aluminum-silicon alloy was melted andchlorine gas bubbled through it. Aluminum Association A-390 alloy hasthe following composmon:

16 to I871 silicon 4-571 copper 0.5 max iron 0.45-0.65 magnesium .271max titanium .171 max manganese .l/( max zinc trace phosphorus balancealuminum The porous carbon crucible was placed in a ceramic crucible andheated to IF. in a nitrogen atmosphere. The molten A-390 was poured overit and the assembly soaked at l 100F. for 30 minutes to equalize thetemperature. Then it was placed in a quick-opening autoclave. Then airat 4200 psi was admitted to the autoclave for two minutes. Thereafter,the pressure was released and the autoclave opened up, and the porouscrucible removed from the molten metal and quenched to room temperature.

Thereafter, the billet was broken out and cut into rough blocks whichwere heated at 935F. for 8 hours and then at 450F. for 4 hours. Theheat-treated blocks were final machined into apex seals.

The material had the following physical characteristics:

apparent density 2.06 gm/cc hardness RG 90 transverse strength 27,000psi resistance .000033 ohm-inches Seals made from this material weretested in a Wankel engine. The engine was a 32 cubic inch engine with an8 to l compression ratio. The seals lasted for 159 hours with a wear of0.0042 inches. The seals broke due to a malfunction of anothercomponent.

The above-described seal material contained approximately 38 percentcarbon by weight. A suitable carbon weight percentage range is 35 to 45percent. The aluminum-silicon alloy and carbon reaction productscomprise the balance. I could not measure the amount of carbide reactionproducts. The aluminum-silicon alloy contained about 17 percent silicon,the desired range being between 14 and 20 percent silicon, from topercent copper or nickel and less than 1 percent other elements selectedfrom magnesium, titanium and iron.

The carbon powder density was 0.8 grams/cc and the alloy density was 2.7grams/cc. The density of the seal material was 2.1 grams/cc. A suitabledensity range is 1.9 to 2.3 grams/cc.

The electrical resistance ranges from 20 to 60 microohm inches. Thetransverse strength should be at least 25,000 psi. The Rockwell Ghardness should be in the range of 80-100.

The foregoing seal material functions as described as a Wankel engineapex seal, a most difficult application.

While the invention has been shown and described in connection with aspecific embodiment thereof, this is intended for the purpose ofillustration rather than limitation and other variations andmodifications of the specific method herein shown and described will beapparent to those skilled in the art all within the intended spirit andscope of the invention. Accordingly, the patent is not to be limited tothe specific embodiment of the method of the invention herein shown anddescribed nor in any other way that is inconsistent with the extent towhich the progress in the art has been advanced by the invention.

I claim:

1. An aluminum carbon composite seal material comprising between 35 andpercent carbon by weight with the balance carbide reaction products andan aluminum-silicon alloy in turn comprising 14 to 19 percent silicon, 2to 5 percent copper or nickel, O to 1 percent magnesium, titanium oriron, balance aluminum, said seal material having the following physicalcharacteristics:

electrical resistance density transverse strength Rockwell G hardness

1. AN ALUMINUM CARBON COMPOSITE SEAL MATERIAL COMPRISING BETWEEN 35 AND45 PERCENT CARBON BY WEIGHT WITH THE BALANCE CARBIDE REACTION PRODUCTSAND AN ALIMINUM-SILICON ALLOY IN TURN COMPRISINF 14 TO 19 PERCENTSILICON 2 TO 5 PERCENT COPPER OR NICKEL 0 TO 1 PERCENT MAGNESIUMTITANIUM OR IRON BALANCE ALUMINUM SAID SEAL MATERIAL HAVING THEFOLLOWING PHYSICAL CHARACTERISTICS,